How do satellites stay up? Orbits explained simply
A satellite is not floating: it is falling around Earth, fast enough that the ground curves away beneath it. What an orbit is, why speed matters, and why low satellites eventually come down.
Falling, but always missing
Throw a ball and it curves back to the ground. Throw it faster and it lands farther away. At about 7.8 km per second, a little over 28,000 km/h, the ground curves away as fast as the ball falls toward it, so it never lands. That is an orbit: a never-ending fall around the planet.
Astronauts on the International Space Station float not because there is no gravity up there (it is about 90% as strong as on the ground) but because they and the station fall together.
Higher means slower
The farther a satellite is from Earth, the weaker gravity is and the slower it needs to move. The ISS, about 400 km up, goes around in roughly 92 minutes, about 16 times a day. GPS satellites, around 20,200 km up, take about 12 hours. At 35,786 km an orbit takes exactly one day, which is why geostationary satellites seem to hang still above one place.
Why low satellites come down
Even at 400 km there is a trace of atmosphere. It drags on satellites a little every orbit, so their orbits slowly shrink. The ISS fires its engines regularly to climb back up; satellites without fuel gradually spiral down and burn up on re-entry.
How quickly depends on the Sun: when it is active, the upper atmosphere swells and drag increases. Above about 800 km a satellite can stay up for centuries.
The shape of an orbit
Six numbers describe an orbit completely. The ones that matter most to a sky-watcher:
- Altitude: how high it flies, which sets how fast it crosses your sky.
- Inclination: the tilt to the equator. The ISS is tilted 51.6°, so it never flies over places beyond about 52° north or south.
- Eccentricity: how stretched the orbit is. Most satellites fly almost perfect circles.